The present invention generally relates to spacecraft ordnance systems and test methods for spacecraft ordnance systems and, more particularly, to an improved spacecraft ordnance system that enables automatic testing of a spacecraft ordnance harness and a method for self-testing of a spacecraft ordnance system.
Spacecraft ordnance systems are explosive release systems that can be used for a variety of pyrotechnic applications such as release systems for antenna tie downs, spacecraft separation devices, mechanism launch locks, and propulsion valves. Spacecraft ordnance systems currently used by Government and industry rely on relatively high electric current to activate these initiators, which require many safeguards to avoid accidentally setting off the initiations.
A typical prior art spacecraft ordnance system 10 is shown in a simplified block diagram in FIG. 1. The spacecraft ordnance system 10 comprises an electrical bridgewire initiation system. For example, a standard initiator 11 (hereafter called “squib”) is connected to a driver unit 13 via a dedicated harness cable 14 comprising a shielded twisted pair of cables. The harness cable 14 is wired to a harness connector 15 providing easy connection between the output of driver unit 13 and the squib 11. Only two squibs, squib 11 and squib 12, are shown in FIG. 1 for simplicity; however, there may be over 100 individual squibs installed on a large spacecraft. The driver unit 13 provides the switching and the current drive necessary to individually fire the squibs. The driver unit 13 comprises multiple switches such that no fewer than three failures can result in an inadvertent squib firing. This is required because squibs are explosive devices and represent a personnel safety hazard. In normal operation, an enable switch 130 is first to be closed, followed by the appropriate arm switch, for example, arm switch 131 arms squib 11, arm switch 132 arms squib 12, etc. The fire switch 133 is then closed, allowing a current of 5 to 6 amperes to flow through the selected squib, causing a small explosive reaction. This in turn allows the mechanism to which the squib is attached (bolt cutter, pin puller, tie down, etc.) to actuate.
Since proper squib firing is absolutely critical to mission success, verification of proper installation of the squib harness cables 14 and 141 in the spacecraft test is also critical. Currently, testing of the spacecraft ordnance system is done manually using a specially designed low current, low range ohmmeter. Test points, for example test points 17, 18 and 19, which allow access to the actual harness wires, are located inside the driver unit 13. To test the continuity of squib 11, for example, the ohmmeter probes are placed on test point 17 and 19. A resistance reading is taken and then manually compared with given pass/fail limits. This resistance measurement process needs to be repeated for each squib circuit. Furthermore, the entire test needs to be repeated several times during the space vehicle integration and test process.
This manual verification process for testing a spacecraft ordnance harness has several disadvantages. It is not possible to unambiguously verify that the proper output of driver unit 13 is wired to the proper squib in the proper location by applying the described test procedure. This can result in squib circuits being swapped or miswired by human error, which can lead to severe on-orbit problems at the point when deployments or propulsion system initializations are performed. Further, the testing of a spacecraft ordnance harness as described above is manual measurement-intensive, and therefore requires a considerable amount of time causing increased cycle time and test cost. Since tests are performed manually, they cannot be performed after the spacecraft is closed out prior to shipment to launch site. The described test for a spacecraft ordnance system also requires specialized test equipment, such as a low-range ohmmeter. Further, the test procedure directly exposes live squibs to potential electrostatic discharge sources, which represents a potential personnel safety hazard. This potential personnel safety hazard is traditionally mitigated by operators wearing ESD grounding protection, but this approach is not foolproof. Also the manual verification process requires the driver unit 13 to be placed outside of the spacecraft for test access purpose exposing the driver unit 13 to a severe environment. If placed inside a spacecraft, the driver unit 13 requires a heavy shielded test access harness, which must fly with the spacecraft, even though the driver unit 13 is only used during testing.
Prior art describes several methods to guarantee that the proper output of a driver unit is connected to the proper squib. For example, color-coding of the squib and the mating harness connector was employed, but this method relies on human judgment and has proven to be ineffective. This method also requires a visual inspection of each mating harness connector; which is not possible after a certain stage in spacecraft-level integration. Pin programming of each squib (i.e. giving each squib its own jumper wire programmed address) was disclosed, but this approach is impractical because it requires the driver unit to interrogate many programming wires from each squib, greatly increasing the wire harness complexity, weight, and cost. Further, mechanical keying of each squib connector was proposed. This is also impractical because it requires a modification to the existing NASA Standard Initiator and therefore increases the cost, requires stocking up to 100 different types of squibs, each with a different key, along with 100 different types of mating connectors, and requires the wire harness designer to have a priori knowledge of the specific key used for each squib at each location. This adds to schedule cycle time and cost. Finally, proposals were made for “intelligent” squibs containing active electronics, in which the squib reports its identity back to the driver unit via a simple digital interface. In addition to being relatively costly, this approach is impractical because of the extremely harsh temperature and radiation environment at many squib locations on the spacecraft. Traditional active electronics are not capable of withstanding these environmental conditions.
There has, therefore, arisen a need for the development of a method for testing of a spacecraft ordnance harness that makes it possible to unambiguously verify that the proper output of the driver unit is wired to the proper squib in the proper location. There has further arisen a need to specify the squibs to allow determination of correct harness routing eliminating the chance of human error. There has also arisen a need to modify the driver unit of the spacecraft ordnance system to enable automatic testing of a spacecraft ordnance harness, to reduce cycle time and test cost, to eliminate the need for specialized test equipment, and to eliminate the potential personnel safety hazard, as connected with manual testing. There has still further arisen a need for the development of an improved spacecraft ordnance system that enables automatic testing of the spacecraft ordnance harness allowing the driver unit to be placed inside the spacecraft where it may be protected from the relatively harsh temperature and radiation environment outside of the spacecraft and allowing the spacecraft ordnance system to be tested at any time during the spacecraft integration and test process, up to and including launch.
As can be seen, there is a need for an improved spacecraft ordnance system that enables automatic testing of a spacecraft ordnance harness and eliminates manual work and human error. Also, there is a need for specification of each squib that allows the determination of correct harness routing. Moreover, there is a need for a method for self-testing of a spacecraft ordnance system providing cost-effective and unambiguous verification that the proper output of the driver unit is wired to the proper squib in the proper location at any time during spacecraft integration, up to and including launch.